Battery systems are gaining increased importance in many areas of technology. A particularly important application relates to electric vehicles, in which the battery systems are a key component for the mobility of the future. A further application of extraordinary importance is that of battery systems as stationary energy storage devices for renewable energies.
Many of the energy storage systems currently in use consist of a series connection or parallel connection of up to thousands of energy storage elements, wherein a relatively small voltage in comparison to the required total voltage is applied, or can be applied to each energy storage element. By using a series connection, the individual voltages add up to produce the total voltage. Using a parallel connection means that the charge is summed. For example, in an electric car such as the Tesla Model S, approx. 6,000 battery cells are installed. The cell voltages and the electrical properties of the cells strongly depend on the cell technology that is used. The cell voltages of typical systems range from 1.0 volts to 3.7 volts.
Due to slightly different physical properties of the cells, they differ with regard to their capacities and their ageing behaviour. Due to the different voltages of the individual cells obtained as a result, in current battery systems it is necessary to balance out the charges of all cells in order to increase the total capacity of the battery system. For performing this balancing, so-called battery management systems (BMS) are currently used, which operate on the basis of an active or passive balancing method. However, such known battery management systems are comparatively expensive, lossy, usually expensive and under certain circumstances even damaging to the cells.
In order to make the energy of the battery system usable for a consumer, a power-electronic converter is also required which is used to stabilize the output voltage or to generate a desired phase of an AC voltage. Furthermore, to charge the battery system a further converter, a so-called charging converter, is usually required.
The state of the art in battery systems has a number of disadvantages. One disadvantage is that the operating points of the system either cannot be adapted to the current requirements at all, or only to a small extent, and that the overall power of the system is typically limited by the weakest sub-unit in the assembly.
In the case of BMS, which are based on a passive balancing of the energy storage cells, energy is knowingly wasted by electrical energy being converted into thermal energy and dissipated. It is also especially the case with passive balancing, that the weakest cell in the assembly determines the total capacity, for example by making the termination of a charging or discharging process necessary.
BMS with more active balancing are usually based on the principle that energy is shifted by charge transfer between cells. This charge transfer, however, is always accompanied by an energy loss and also reduces the service life of the cells.
In the conventional systems it is typically also necessary that all cells in the system are of the same type and have the smallest possible differences in their electrical and physical characteristics. In addition, current systems typically operate with high technical circuitry and filter complexity, which increases the energy consumption and the costs.
Similar problems to those in battery systems also occur in energy conversion systems, which comprise, for example, fuel cells or solar modules as the energy conversion elements. In energy conversion systems of this type also, a plurality of cells are connected in series to increase the total voltage and connected in parallel to increase the charge or the current flow.